1. Field of the Invention
The present invention relates to a method of controlling wavelength used in wavelength multiplex communication networks and also to a network system using the wavelength controlling method.
2. Description of the Related Art
In general, a wavelength multiplex communication network system employs a multiplicity of independent channels arranged in a single transmission line such that each channel performs communication independently of other channels. Such a wavelength multiplex communication network system does not necessitate frame synchronization or other multiplexing processing along the time axis and, hence, does not require coincidence of transmission rates of different channels. This type of communication network system, therefore, is suitable for use in multi-media communications in which flexibility or adaptability of the network is an important requisite.
A wavelength multiplex communication network system has, for example, a number of terminal stations, each having a combination of a wavelength tunable optical transmitter and receiver. In operation, the transmitting terminal station controls the wavelength tunable optical transmitter such that the transmitter transmits light of a wavelength which is not being used on the network communication transmission line, while the receiving station controls a tunable optical filter of the optical receiver such that the central wavelength of the spectrum passed by the filter coincides with the wavelength of the light to be received. Thus, the range of wavelength which can be used in the wavelength multiplex communication network system is governed by the ranges over which the wavelengths of the optical transmitter and receiver are variable. Meanwhile, the wavelength spacing of the channels (referred to as "channel spacing", hereinafter) is determined by the width of the spectrum transmitted through the optical filter of the optical receiver.
A semiconductor laser (referred to as "LD", hereinafter) of the wavelength tunable type can be used as the wavelength tunable light source. Research in recent years has been aimed towards widening the range of wavelength variation. At present, LDs of the multi-electrode DBR (Distributed Bragg Reflector) type and the multi-electrode DFB (Distributed Feedback) type have been put to practical use. These LDs have wavelength tunable ranges on the order of several nanometers (nm). An example of such multi-electrode LD is shown in TRI-ELECTRODE RESONATOR .lambda./4 SHIFT MQW-DFB LASER, Technical Report of the Institute of Electronics, Information and Communication Engineers, OQE (Optical and Quantum Electronics), 89-116. The tunable optical filter may also be a Fabry-Perot resonator-type filter. This type of filter, which has been put to practical use, has a wavelength tunable range of several tens of nm and a spectral width of 0.1 nm. An example of this type of filter is shown in "A field-worthy, high-performance, tunable fiber Fabry-Perot filter" preprint, ECOC (European Conference on Optical Comunication) '90-605.
The number of channels in a wavelength multiplex communication network system having a given range of wavelength variation can be increased by reducing the channel spacing. In order to reduce the channel spacing to a value which is not greater than the width of fluctuation caused by drifting of wavelengths of the tunable LD and tunable optical filter, it is necessary that the influence of drift is suppressed by a suitable control. To this end, it is necessary to stabilize the wavelength both absolutely and relatively. However, it is not easy to obtain an absolute reference for wavelength. Relative wavelength stabilization is also difficult to achieve, particularly in a communication network such as a LAN in which optical transmitters are located at remote places.
The state of the art is such that the continuous wavelength tunable range afforded by tunable optical filters is greater than that of tunable LDs. For instance, while the continuous wavelength tunable range of a typical tunable LD is on the order of several nm, the range over which the wavelength transmissible through the tunable optical filter well exceeds 10 nm. Therefore, in a system in which tunable LD is used as light-emitting means while a tunable optical filter is used as detecting means, the continuous wavelength tunable range of tunable LD is one factor limiting the number of the channels.
It has been known that there is a type of light-emitting means which can materially expand the range of wavelength variation of the emitted light, on condition that discontinuities of wavelength are allowed to be present in the wavelength tunable range. For instance, a certain type of tunable LD can exhibit an expanded wavelength tunable range of 10 nm or greater when discontinuities of wavelength are permitted. An example of such tunable LD is disclosed in IEEE Photonics Technology Letters, "A Three-Electrode Distributed Bragg Reflector Laser With 22 nm Wavelength Tuning Range," Vol. 3, No. 4, pp 299-301, (1991). More specifically, this LD is a three-electrode DBR-LD which has a gain-current supply electrode, a phase-current supply electrode and a Bragg-current supply electrode. It is reported that an to expanded range of luminescent wavelength spanning about 22 nm between 1512 nm and 1534 nm, when the current supplied to the Bragg electrode is varied between -120 mA and +120 mA, while the gain-current supply electrode is supplied with a constant current of 175 mA. It is also reported that there are about 20 discontinuities within the above-mentioned expanded range.